1. Field of the Invention
The present invention relates to an optical amplifier and a method of controlling an optical amplifier in which a gain within a present amplification signal band is initially set based on an ASE (amplified spontaneous emission) optical power.
2. Description of the Related Art
Conventionally, in an optical amplifier using an optical fiber as a medium that amplifies an optical signal, there is a correlation between a gain and an ASE light. That is, an ASE light power Pase and a gain G have a relationship of Pase=K×G. Herein K is a proportional constant between the ASE light power and the gain. Hereby in some optical amplifiers, a pumping light power or an ASE light power is controlled by monitoring the ASE light power to calculate a gain so that the calculated gain becomes a setting value (see Japanese Laid-open Patent Publication No. H6-21582).
For example, in a case of a Raman amplifier, a Raman gain is indicated as a ratio of a signal light power when a pumping light is on relative to a signal light power when the pumping light is off. However, the Raman gain cannot be monitored in an amplification operation since the pumping light cannot be off in the amplification operation. In such a case, a gain of the Raman amplifier can be controlled by monitoring an ASE light to indirectly obtain a Raman gain by using the above-described proportional relationship and by controlling a pumping light power so that the Raman gain becomes a setting value.
When a pumping light is input into an optical fiber of a Raman amplifier, a gain is obtained at a position shifted to a longer wavelength side relative to that of the pumping light by approximately 100 nm. A Raman gain or a tilt which is a wavelength dependency of the Raman gain is controlled by controlling the pumping light power (see U.S. Patent Application Publication No. 20110141552).